Studies on Rubber Composition as Passive Acoustic Materials in Underwater Electro Acoustic Tranducer Technology and Their aging Characteristic

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STUDIES ON RUBBER COMPOSITIONS AS PASSIVE ACOUSTIC MATERIALS IN UNDERWATER ELECTRO ACOUSTIC TRANSDUCER TECHNOLOGY AND THEIR

AGING CHARACTERISTICS

Thesis submitted to the

COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY

by

VELA YUDHAN BALAKRISHNA PILLAI

ill partial fUlfillmell t of the requiremellts for the award of the degree of

DOCTOR OF PHILOSOPHY

~AVAL PHYSICAL AND OCEANOGRAPHIC LABORATORY KOCHI-682021

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CERTIFICATE

This is to certify that this thesis entitled

"Studies

on rubber compositions as passive acoustic materials in underwater eleetTo acoustic transducer technology and their aging characteristics" is a report of the original work carried oul by ShTi. Velayudhan Blalakrislllla Pil/ai. in Naval Physical and Oceanographic Laboratory, Kochi. No part of this work reported in this thesis has been presented for any other degree/rum any other institution.

Kochi-21 I, October 2003

Dr. J . Narayana Das Scientist G

NPOL, Kochi

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Preface

Rubber has become an indispensable material in Ocean technology.

Rubber components play critical roles such as sealing, damping, environmental protection, electrical insulation etc. in most under water engineering applications. Technology driven innovations in electro acoustic transducers and other sophisticated end uses have enabled quantum jump in the quality and reliability of rubber components. Under water electro acoustic transducers use rubbers as a critical material in their construction.

Work in this field has lead to highly reliable and high performance materials which has enhanced service life of transducers to the extent of 10- 15 years. Present work concentrates on these materials. Conventional rubbers are inadequate to meet many of the stringent functional of the requirements. There exists large gap of information in the rubber technology of under water rubbers, particularly in the context of under water electro acoustic transducers. Present study is towards filling up the gaps of information in this crucial area.

The research work has been in the area of compounding and characterisation of rubbers for use in under water electro acoustic transducers. The study also covers specific material system such as encapsulation material, baffle material, seal material, etc. Life prediction techniques of under water rubbers in general has been established with reference to more than one functional property.

This thesis is divided into 6 chapters.

Chapter 1 presents a review of under water engineering rubbers with particular reference to rubbers used in electro acoustic for electro acoustic transducers.

Chapter 2 deals with theoretical studies on rubber compounding, compounding materials and their characterisation techniques.

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Chapter 3 covers the experimental studies on diverse parameters affecting water intake through diffusion and permeation mechanism. This also deals with specific material system for encapsulation, baffles, seals etc.

Chapter 4 presents theoretical investigation on aging and life prediction

,

techniques as well as investigation on Viscoelastic parameters of under water rubbers.

Chapter 5 gives a consolidation of experimental results and the observations are discussed for better understanding of the underlying phenomena emerging out of theoretical and experimental works covered in Chapters 3 and 4.

Chapter 6 presents important conclusions arrived at as a result of the research work.

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n

2.8 Thermal analysis of Rubber 78

2.8.1 Thermogravimetric Analysis (TGA) 78

2.8.2 Differential Scanning Calorimetry (DSC) 79

2.8.3 Modulated DSC (MDSC) 80

References 82

Chapter 3 Compounding Studies 85

3.1 Introduction 85

3.1.1 Sample preparation 86

3.2 Neoprene 86

3.2.1 Water absorption behavior of neoprene gum vulcanisate 86 3.2.2 Kinetics of water absorption in gum neoprene vulcanizates 90 3.2.3 Water absorption behaviour of neoprene rubber

compound with different concentrations of GPF black 93

3.2.4 Effect of Salinity 96

3.2.5 Permeability of neoprene gum rubber 103

3.3 Bromobutyl Rubber 106

3.3.1 Diffusion behaviour of Bromo butyl rubbers 106 3.4 St~'rene Butadiene Rubber (SBR) 107 3.4.1 Kinetics of diffusion processes of water in SBR rubber 107

3.5 Natural Rub~r 108

3.5.1 Water absorption behaviour of natur,ll rubber 108

3.6 Curing ~eha\'i()ur of Rubbers 110

3.6.1 Kinetics of modulus build up durin~ cure 111

3.6.2 Cure Studies 112

3.6.3 Effect '.': diff<.>rent types of carbon btlCk on the cure reactions 114

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3.6.4 Effects of furnace black on the curing reactions in

neoprene rubber with varying quantities of red lead 116 3.6.5Comparative study on zinc oxide v red lead curatives on the

physical properties of neoprene rubber 116

3.7 Low Temperature Vulcanization 119

3.7.1 Structure and properties of Poly ethylene imine 120

3.7.2 Preparation of Samples 120

3.7.3 Cure Studies using Moving Die Rheometer (MDR 2(00) 121 3.7.4 Cure Studies Using Modulated Differential

Scanning Calorimetry (MDSC) 121

3.7.5 Cure kinetics using DMA 121

3.8 Influence of carbon black on Dynamic Mechanical behavior 127 3.8.1 Influence of the type of carbon black 127

3.8.2 Effect of concentration 132

3.8.3 Acoustic properties 140

3.9 Application specific studies 146

3.9.1 Water Absorption Behavior of Encapsulation Rubber 146 3.9.2 Comparison of water absorption

characteristics of different rubbers 148

3.9.3 Absorption of moisture from humid environment 149 3.9.4 Influence of inert environment on water absorption 150 3.9.5 Effect of different quantities of fillers on volume resistivity

of different rubbers 151

3.9.5 Acoustic Baffle 154

3.9.6 Stvdies on underwater seal composition 163

References 173

Chapter 4 Aging Studies 175

4.1 Aging phenomena 175

4.1.1 Controlling Parameters 177

4.1.2 Accelerated Aging 178

4.1.3 Aging models 179

4.1.4 Ihermogravimetric degradation and kinetics 183

4.1.5 Water absorption and Permeation 186

4.2 Effects of aging on properties and Life prediction 192

4.2.1 Ultimate elongation 192

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4.2.2 Compression stress relaxation 4.2.3 Electrical properees

4.3 Influence of water exposure on aging

4.3.1 The Influence water immersion and humidity 4.3.2 Electrical Resistivity

4.3.3 Dynamic Mechanical Properties

]96 198 200 200 202 203 4.3.4 Influence of hot humid aging on the transient response '206 4.3.5 Influence of wet aging on the kinetics of thermal degradation 217 4.3.6 Analysis of glass transition and clustered water 223 ference

lapter 5 Results And Discussion 5.1 Compounding considerations 5.2 Water intake bv rubbers

5.3 Electrical resistivity considerations 5.4.Transducer Encapsulations 5.5 Curing Process and Accelerators 5.6 Low Temperature Vulcanisation 5.7 Viscoelastic Behavior

5.8 Acoustic Baffles 5.9 Underwater seals

5.10 Aging and Life Prediction ferences

lapter 6 Summary And Conclusions 6.1 Summary

6.2Conclusions ope For Further Work

st of Abbreviations and Symbols st of Compounding Ingredients used

st of Publications and Patents emerging from the work

227 231 231 232 237 238 240 241 243 246 248 249 255 257 257 262 265 267 273 275

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Chapter'1

INTRODUCTION

The use of rubber in engineering applications dates back to mid nineteenth century. In 1850 rubber springs were used for dray wagons. A more spectacular application is the use of 12 mm thick rubber blocks in 1889 to support a railway viaduct in Melbourne. Today the original Natural Rubber (NR) parts are still functioning. Innovations driv~n by technological requirements and environmental protection issues have led to several strides in rubber technology. Some of the most striking examples of such innovations relate to applications in offshore engineering and underwater acoustic systems. Offshore oil platform design has transformed from rigid towers of steel and concrete to compliant structures capable of deep-water oil production. Elastomeric flex joints form an intrinsic part of the structural design. The ride comfort and safety of modern passenger vehicles, trucks etc. depend on advanced suspension systems, which often use a large number of rubber components. Modern aircrafts, helicopters and the space shuttles use engineered rubber components for key functions, where the failure may be catastrophic. Ocean environment is extremely aggressive in terms of marine corrosion, salt laden atmosphere, waves, tides, ocean currents, temperature variations, pressure variations etc. Particular merit of rubber is its ability to function in such aggressive environn1ents for long years. A unique feature of rubber is very low shear modulus which. is more than a thousand times lower than its bulk modulus and its ability to deform elastically several hundred percent. A wide range of engineering applications has been developed on the basis of these properties. Rubber components capable of very different stiffness in

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Chapter 1

different directions have been designed. In structural engineering they find applications such as bridge bearings, compliant foundation' agamst seismic disturbance, offshore mooring system assemblies where flexible connections are provided by rubber flex joints etc. Rubber plays crucial roles in underwater acoustic- systems, such as acoustic windows, acoustic baffles encapsulation materials, shock and vibration absorbers. Rubber components are extensively used as seals, energy absorbers, environmental protection materials, electrical insulation etc in most underwater engineering applications. The range of versatile applications has been possible due to the large scope for tailor making of properties in rubber compounds.

1.1 Rubbers in underwater Engineering 1.1.1 General considerations

Rubbers are used in a verity of components and devices starting from rubber seals, to sonar dome baffles. Performance requirements are specific to each component. For example, for a deepwater seal retention of sealing efficiency demands compression stress relaxation to be minimum. On the other hand for anti vibration applications damping behaviour is paramount. For rubbers used in acoustic applications, wave propagation characteristics assume greater significance. Rubbers employed for encapsulating electro acoustic transducers must have both electrical and acoustic properties as desired. However, in all underwater applications, the interaction of rubber with the water medium, determines the reliability and service life. Reliability is always the over riding priority.

1.1.2 Mechanical Properties

MechanicqI properties of rubbers are unique. Stresses, for example are depenl1ent on time and temperature and non linear with strain.

Rubbers also have unique Poisson's ratio, frictional properties, Gough- Joule effect, and very high -energy storage. Hysteresis is high.

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Introduction Rubber and rubber products are tested for initial and continued performance of the unique functions that are enabled by their viscoelastic properties.

Static or low deformation tests for the common stress -strain properties of modulus, tensile strength and elongation are usually made only for quality control since rubber products are seldom elongated in service. Low rate tests for creep, stress relaxation, and set, reflect the limitation of service because time dependent properties are peculiar for rubbers because of the time dependent properties are peculiar to rubbers.

Examples are anti vibration mounts, seals and gaskets. The amount of static deformations also affects such other properties penneability, electrical properties, and low temperature behavior. Flexibility is important in coatings, encapsulation materials, acoustic baffles transducer mountings etc. Hardness is a function of modulus.

Elastomers are usually deformed dynamically in service.

Deformation is thus resisted by elastic and damping forces. The principal purpose of dynamic mechanical tests is to evaluate these forces and attendant loss of energy.

Physical testing of rubber often involves application of a force to a specimen and measurement of the resultant deformations. Two common modes are tensile and shear. Concept of stress and strain is used to denote the results. Stress is the force per unit cross sectional area i.e., F / A Strain is the deformation per unit original length (.6.L/Lo) in tensile tests or deformation per unit distance between contacting surfaces (S/D) in shear tests. Stress is expressed as Pascal (Pa). Strain is often expressed as a percentage rather than a ratio. The term extension ratio is the length at a given point in,the test sample divided by its original length Young's modulus E is given by

FIA

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Chapterl

Same equation applies when a bar is decreased in length by a compressive force. The rigidity or shear modulus G is defined as the ratio of shearing stress to shearing strain:

G= FIA

SID 1.2

A third type of modulus is the bulk modulus B. It is defined as the ratio of hydrostatic pressure to volume strain.

B

=

Hydrostatic Pressure Vol.change/unit Volume

When a material is stretched its cross sectional dimensions decrease Poisson's ratio v is the constant relating these change in dimensions;

Changein width per unit of width

v

=

---=---..:.---

change in length per unit of length

The stress-strain test in strain is the most widely used test in rubber industry. Among the purpose of the tests are to determine effects of liquid immersion on aging, to ensure that all compounding ingredients have been added in proportions, to determine rate of cure and for optimum cure for experimental compounds. Standard methods for tensile testing of vulcanized rubbers are given in ASTM-0-412

1.1.3 Electrical Properties

Underwater electro acoustic transducer as the name implies are

~Iectrical devices, often handling heavy power in kilowatt ranges. Rubber :omponents used in their construction such as encapsulation, cable chains, :onnectors etc should have adequate electrical insulation characteristics.

Permanence of .. insulating capacity under long water submergence :onditions is an essential requirement. In rubbers electrical insulation )ehaviour is largely a function of compounding ingredients and their iistribution within rubber itself. Design of the material must ^caterfor high

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Introduction and stable electrical resistivity. Effect of different types of carbon black and their quantity needs to be known for designing rubbers with optimum electrical behavior

1.1.4 Viscoelastic Properties

Rubber molecules are characterised by long chains with a coiled morphology in an unstrained condition. When stressed, these coils unfurl and give rise to large strains before rupture. Within elastic limits the molecular chains retract to at equilibrium once external load is removed. In the case of an ideally elastic material the strain goes to zero. An ideal linear elastic solid obeys Hooks law: which can be written in the form

F= kx (1.3)

Where F is force, x is deformation and k is the spring constant.

On the other hand, in a purely viscous material, the molecules rearrange in the new geometry permanently. An ideal viscous liquid obeys Newton's law: stress is proportional to rate of change of strain with time.

Newton's law may be written in the form:

"{ =

f\c(dyjdt) (1.4)

f\c is a viscous damping coefficient and dy j dt is the shear strain rate.

The laws above may be written alternatively as o=G£

for an elastic solid and

0= f\e{d£jdt) for a Newtonian fluid.

(1.5)

(1.6)

where 0 is shear stress, £ is strain, G is the modulus and lle is the Newtonian viscosity. Beha~iour of rubber is a combination of such ideal elastic solid and a purely viscous fluid.

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Chapter:!

Same equation applies when a bar is decreased in length by a compressive force. The rigidity or shear modulus G is defined as the ratio of shearing stress to shearing strain:

G= FIA

SID 1.2

A third type of modulus is the bulk modulus B. It is defined as the ratio of hydrostatic pressure to volume strain.

B

=

Hydrostatic Pressure Vol. change I unit Volume

When a material is stretched its cross sectional dimensions decrease Poisson's ratio v is the constant relating these change in dimensions;

Change in width per unit of width v

=

----=---...::.--- change in length per unit of length

The stress-strain test in strain is the most widely used test in rubber industry. Among the purpose of the tests are to determine effects of liquid immersion on aging, to ensure that all compounding ingredients have been added in proportions, to determine rate of cure and for optimum cure for experimental compounds. Standard methods for tensile testing of vulcanized rubbers are given in ASTM-D-412

1.1.3 Electrical Properties

Underwater electro acoustic transducer as the name implies are electrical devices, often handling heavy power in kilowatt ranges. Rubber components used in their construction such as encapsulation, cable chains, connectors etc should have adequate electrical insulation characteristics.

Permanence of.. insulating capacity under long water submergence conditions is an essential requirement. In rubbers electrical insulation behaviour is largely a function of compounding ingredients and their distribution within rubber itself. Design of the material must cater for high

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Introduction and stable electrical resistivity. Effect of different types of carbon black and their quantity needs to be known for designing rubbers with optimum electrical behavior

1.1.4 Viscoelastic Properties

Rubber molecules are characterised by long chains with a coiled morphology in an unstrained condition. When stressed, these coils' unfurl and give rise to large strains before rupture. Within elastic limits the molecular chains retract to at equilibrium once extemalload is removed. In the case of an ideally elastic material the strain goes to zero. An ideal linear elastic solid obeys Hooks law: which can be written in the form

F=kx (1.3)

Where F is force, x is deformation and k is the spring constant.

On the other hand, in a purely viscous material, the molecules rearrange in the new geometry permanently. An ideal viscous liquid obeys Newton's law: stress is proportional to rate of change of strain with time.

Newton's law may be written in the form:

1:

=

flc(dy/dt) (1.4)

fie is a viscous damping coefficient and dy / dt is the shear strain rate.

The laws above may be written alternatively as o=G(

for an elastic solid and

0= fle(d(/dt) for a Newtonian fluid.

(1.5)

(1.6)

where 0 is shear stress, ( is strain, G is the modulus and fie is the Newtonian viscosity. Beha~iour of rubber is a combination of such ideal elastic solid and a purely viscous fluid.

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Chapter 1

This behaviour can be represented as

0= GE + lle(dE/dt) (1.7)

This combination viscous and elastic behavior makes rubbers viscoelastic materials. Viscous properties are desired in rubbers for shock absorption and acoustic damping. Many practical problems like stress relaxation creep, compression set etc are manifestations of viscous properties.

Underwater acoustic transducers are typical dynamic devices operating over a wide frequency band and temperature range. Since acoustic wave imposes periodic stress in material mechanical properties becomes a function of exciting wave. The term dynamic mechanical properties refer to the behaviour of materials when subjected to stresses and strains that change with time. Traditionally viscoelastic behaviour has been described by means of Maxwell and Voigt elements (Fig.1.1and Fig.1.2)

E

Fig.l.l Picture presentation of Maxwell model

~

^!

E

Fig.l.2.Picture presentation Voigt model

Most materials exhibit behaviour that is more complex than either of these two simple models.

The Voigt element in terms of stress and strain is written as (Eqn.1.7) A sinusoid ally varying strain can be expressed as

G= E" sinwt ( 1.8)

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Introducing the complex notations,

G= £oexp(i(J)t) = £o(cos (J)t + i sin(J)t) where i

=

^(_1)'h

Rate of change of strain with time is given by dEI dt

=

^i(J)^£0exp(i(J)t)

=

^i(J)E

Substituting in the equation, we get, o

=

(G+ i(J)ll) E

Introduction

(1.9)

(1.10)

(1.11) The term in parenthesis represents modulus because it is a ratio of a stress to strain. It is denoted by the complex dynamic modulus G:

Denoting Gl is the real part of the complex number. Likewise, wll is defined as the imaginary dynamic modulus given by symbol G2.

Thus these terms are rewritten as follows:

0=( Gl+i G2) E =G*E (1.12)

Eqn. 1.12 defines real, imaginary and complex modulii. Absolute value of complex modulus is given by the ratio of stress amplitude aa, to strain amplitude £0.

Thus, G* = 00/ £0

=

^(Gl2 +G22)'h= GI[l + (tan 6)2]'1, Where tan 6 is the ratio G2/GI.

(1.13)

A typical dynamic mechanical evaluation involves applying a sinusoidal stress or strain to sample and measuring the resulting strain or stress. Because of the energy dissipation in material the stress and strain will not be in phase. The lag between the two is monitored and calibrated to yield elastic and viscous components of the complex modulus. The complex elastic modulus will have two components

As shown in Eqh.1.13, we have

G*=G' + iG" (1.14)

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Chapter 1

Real part of the complex modulus denotes the energy stored in the material due to its elastic nature, whereas the imaginary part corresponds to the energy lost due to viscous behaviour. Ratio of the two components is the mechanical loss factor of the material

tan 0 =G"/G' ( 1.15)

o

is the angle by vvhich the strain lags behind the stress in such cases.

Polymers are characterized by wide variation of stiffness and damping properties with frequency and temperature. Thus it is imperative that they must be optimized for specific dynamic mechanical behaviour within the operating band. Selecting a rubber for a particular application requires an accurate knowledge of its dynamic mechanical properties.

Like other functional behaviour, acoustic behaviour also undergoes drastic change around glass transition zone, in polymer. Glass transition in amorphous polymer materials marks the onset of cooperative thermal motions of individual chain segments, involving large scale conformational rearrangements of the chain backbone. Below the glass transition large scale motion becomes frozen. Major changes in physical properties, including acoustic properties, take place at the glass transition. According to the free volume theory [1] the total macroscopic volume of a polymer is considered to be the sum of the actual volume of the polymer chains (the occupied volume) and the holes or voids that constitute the free volume.

Glass transition occurs when there is enough free volume for large scale molecular motions associated with the transition to take place. It is assumed that the occupied volume increases linearly with temperature throughout but free volume undergoes a discontinuous increase in expansion coefficient at the glass transition when the number of holes increases. A number of factors have been shown to influence glass transition temperature(Tg), such as backbone flexibility, steric effects, polarity, pendent groups, crystallinity, presence of plasticizers, cross link density etc.

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Introduction Table11Glass transition temperatures of some common rubbers

.

:- Rubber Formula Tg (0C)

Natural Rubber -CH~C(CH3)=CH-CH2- -72

Polychloroprene -CH~C(Cl)=CH-CH2- -50

,

styrene butadiene Rubber [-CH~CH=CH-CH2-CH2=CH(CH5) ] -65

I

Butyl Rubber -(CfuhC-CH2-CH2-CCfu=CH-CH~ -70

Nitrile rubber [-CH2-CH =CH-CH2-CH2-CN] -20

Poly cis-l,4-Butadiene [-CH~CH=CH-CH2]- -108

Polydimethyl Siloxane (Cfu)Si-O-Si(Cfu)-0- -123

The table illustrates the effect of some of the factors that influence the glass transition. Poly dimethyl siloxane has the lowest Tg because of two flexible backbone components. Comparing Poly butadiene with Natural Rubber, it is seen that substitution of a methyl group for a hydrogen atom raises Tg as a result of the steric hindrance caused by the larger size of the methyl group. Steric effects are important because glass transition requires certain free volume to be available. Comparing natural rubber with Polychloroprene the methyl group is comparable in size to chlorine atom but the Tg of neoprene is higher because of the greater polarity of the chlorine atom. Polarity increases inter chain attraction, which decreases free volume and hence raises Tg. In SBR copolymer large pendent phenyl group in styrene is effective in raising Tg of copolymer to -65°C though Tg of poly butadiene itself is -108°C. Other factors influencing the glass transition are, cross link density, Co-polymerization and additives such as fillers and plasticizers.

The linear viscoelastic properties of polymers are both time and temperature dependent. For underwater transducer applications visoelastic

,.

properties of interest is spread over several decades of frequency.

Experimental estimation of viscoelastic properties over bands of

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Chapter 1

technique. This technique is based on an empirical relationship between the time and temperature dependent properties of viscoelastic materials. The relaxation process of a polymer at a particular temperature will be enhanced at elevated temperature i.e. the relaxation time will be shorter at any higher temperature. In essence, the time- temperature superposition principle assumes that by changing the temperature the complete relaxation spectrum is affected to the same degree. Hence increasing temperature shortens all relaxation times by the same factor. In the actual experiment the temperature is held constant, the frequency and time is varied. By repeating the experiment at different temperature a set of isothermal dependencies of E' or E" on frequency, ^(j)can be obtained. Any of the viscoelastic parameters can be shifted along the time/ frequency axis such that they are superposed on one another to generate a master curve at a particular temperature.

According to amount of shift of a frequency scan that is associated with a particular temperature will b~ different from that of a frequency scan associated with any other temperature [2-7]. Therefore for every temperature, there is a characteristic shift-factor. The William-Landel-Ferry (WLF) equation [2,8]

1.1.5 Underwater properties

Diffusivity of water in rubber, presents a picture much different from that of other liquids. Absorbed water affect physical, mechanical and electrical properties of rubbers. Rubber compounds contain varying quantities of water-soluble ingredients that act as sinks which absorb water as it diffuses through the rubber phase. The process is diffusion controlled.

Diffusion of water through rubber is an important consideration in underwater application [9,10,11]. A number of theoretical treatments have appeared in the literature on the problem of estimating diffusion coefficient from absorption data [12-14]. Tester [15] derived an equation from the consideration of an osmotic mechanism to fl'present absorption for

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Introduction vulcanised rubber immersed in water. The relationship was tested on a vulcanised rubber and the results are in good agreement with theory in the absence of air. Deviations found in the presence of air were explained to be caused by aging resulting from oxygen dissolved in water. Accelerated aging was shown to be responsible for the marked increase of the rate of absorption. Aminabhavi et al [16] suggested simple ways by which diffusivity could be determined from water absorption experiments. They estimated activation energy of diffusion processes in eR, SBR and EPDM rubber. Their study showed lower diffusion coefficient for all materials in salt water as compared to distilled water though solubility coeffident(s) and permeability (p) are higher. Amerongen [17] reviewed diffusion of matter in rubbers. The survey covered the fundamental background, including mathematical models required to under~tand pxperimental approach to diffusion, and for interpretation of measurements. The subjects discussed include diffusion, solubility and permeation of organic liquids, water and solids such as sulphur and other compounding ingredient in and through rubber. Paper also discusses temperature dependence of diffusion;

effects of modification of the rubber vulcanisation, crystallisation etc which are of great relevance in the present study.

Theoretical aspects of diffusion have been well studied since Ficks [18] laid foundation in 1855. Solution to the diffusion equation has been obtained for simple geometric shapes (cubes, rod, sheet etc)[12-14].

Mathematical solution based on the serni- infinite medium has been found to be adequate for many situations [19]. In this case it is assumed that the liquid extends to infinity on one side of the interface and that rubber extends on the other side. In a sheet of finite thickness diffusion behavior is the same as this until the liquid reaches the centre of the sheet. When a plane polymer sheet is exposed to fluid, the changes in concentration (C) of the diffusing substance as a function of time (t) and position (x) is given by

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Chapter 1

(1.16)

where 'D' is the diffusion coefficient. If the material has a uniform initial diffusion concentration (Co) and the surface is kept at a constant concentration (C), the solution [12-14] of Eqn.1.1 is

C-Co/C,-Co=l-4/Tt

L::=o

^[(-1)^{n /} ^(2n+1)]

x exp [-D(2n+1)2Tt2 tjh2] cos (2n+l)Tt x/h (1.17) where' n' is an integer from 0 to 00. The total amount of substance diffusing into the polymeric material (Mt) as a function of time is given by the integral of the Eqn. 1.17 across the thickness (h):

where Moo is the equilibrium value of the diffusing substance at infinite time and is estimated from the diffusion plots by extrapolation. The diffusion process will show an initial linear increase and then asymptotic saturation associated with plasticization effects.

For long times, Eqn.1.18 may be approximated by

(1.19 ) and the approximation for short time is:

Mt/ Moo=4/h[Dt/ Tt ]1/2 (1.20)

Eqn.(1.20) is a valid representation of the time dependence of the water uptake.

It follows from ~qn.1.20 that 0 = n:[h/4M",P [Mt/ t

Ihp

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Introduction When diffusion proceeds, the boundary between swollen and unswollen matrix advances with a rate 'P'. P is related to the diffusion coefficient 'D', by the relation [20]

P

=

..J(4D/1t (1.21)

Eqn.l.20 implies that swollen boundary movement is proportional to square root of time.

Diffusion of water in rubber is complex due to hydrophilic impurities present in rubber [20,21]. Several workers [22-24] studied the osmotic effects in water absorption. Fedors [22] described the absorption of liquids by polymers which contain liquid soluble inclusions. It was shown that the equilibrium uptake of water can be calculated if several properties of the inclusion such as solubility as well as modulus of thf> polymer were known.

Muniandy and Thomas [21] tested the hypothesis that hydrophilic impurities cause rubber vulcanisates to absorb high amount of water than pure hydro carbon rubbers and time to reach equilibrium absorption in such cases is very high. They carried out experiments on model compounds incorporating known amount of sodium chloride in a pure rubber. Based on the results they developed a theory to explain the amount and rate of water absorption in rubber vulcanizates. Hydrophilic impurities may originate from natural sources in natural rubber and from emulsifying agents and catalysts in the synthetic rubbers. Water diffuses through rubber phase in which it is only slightly soluble and collects around the hydrophilic impurities fOrming droplets of solution. The droplets of solution will exert osmotic pressure on the rubber, which acts as semi permeable membrane.

Resulting elastic stress in the rubber, arising from enlargement of the cavity containing the impurity will resist dilation. When elastic forces are sufficient to balance the osmotic pressure no further enlargement of the droplet will ocellI. The equilibrium condition is given by

1to = 1tI-pr (122)

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where 1to is the osmotic pressure of the external solution in which the rubber is immersed 1tl is the osmotic pressure of the droplet solution and pr is the elastic pressure exerted on the droplet by rubber.

Classically the osmotic pressure is given by

1t=CRT/M (1.23)

where C is the concentration of the solute of molecular weight M. R is the gas constant, T the temperature.

Elastic pressure prl exerted on the spherical cavity in an infinite block of rubber is given is given by

pr= E/6[5-4/A-1/1.,,4] (1.24)

Where E is the Young's modulus of the rubber and)", is the extension ratio of the rubber at the surface of the cavity.

1to-1tl

=

E/6[5-4/)"'-1/)...4] (1.25) Impurities act like sinks of water, producing pockets of higher concentration within the rubber matrix which will reduce the rate of diffusion.

The study of water permeation through elastomers has attracted much attention owing to its wide technical applications [13]. The aim of such studies has been to collect information to the packaging industry, to develop liquid - liquid separation processes or to study diffusion mechanisms and morphology of polymer membranes. Water permeation through elastomers and plastics have been comprehensively reviewed by Cassidy and Aminabhavi [25]. They summarized the available data for the period from1968 t01982 on, diffusivity, solubility and permeability of water and water vapour into and through elastomers and plastics. Kosyanon and McGregor [26] analysed diffusion data from literature and found that the diffusion coefficient of gases in elastomers can be accounted for by the WLF equation [8]. Parameter K=Bd/Bf of Frisch and Rogers [27] is used as a

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Introduction

ti'on factor K and log (Og) are shown to vary with the penetrant. From

correc .

the values of K and log Dg of the gases their diffusion coefficient in any elastomers of known Tg could be estimated. From Arrhenius equation and the WLF relationships, an equation· is derived to predict the activation energy of diffusion directly from temperature at which diffusion is taking place, Tg of the polymer, value of K and universal constants A and B. Cassidy [28] and others have studied several elastomers such as neoprene, styrene butadiene rubber, nitrile rubber and their binary composites for fresh water and salt water permeation. Diffusivity data have also been collected on a few elastomers. Arrhenius activation parameters for the transport processes involved in the experiments have been estimated at 3 temperatures. Cassidy and Aminabhavi [29] in another work studied permeability of SBRjEPDM single and binary laminates for directional flow behaviour of distilled water and salt water. Result of the study supported activated transport mechanism.

Under activated transport mechanism temperature increase causes water absorption rate to increase. For all elastomers, salt water exhibited higher permeation rates than did distilled water. Distilled water showed greater directional behaviour compared to salt water. In most cases it was found that diffusion coefficient D decreases as the total concentration C of water absorbed is increased. The effect is more pronounced in elastomers containing water- soluble salts such as sodium chloride. Barrie et.al [30]

measured the sorption, diffusion and permeation of water in cis poly isoprene and natural rubber. They found that at higher relative pressures the diffusion coefficient in all cases decreased with concentration and activation energies for diffusion increased with concentration. Their findings were to be expected since with increased clustering of the sorbed penetrant increases at higher relative pressures.

Molecular. diffusion of water through elastomers is an important consideration in underwater applications. In most practical situations

(26)

Chapter 1

(1.26) Wand Wd are respectively the wet and dry weight of the polymer membrane. "Vhen a material is immersed in a liquid, absorption takes place and this process can be described by relation (1.19)

Diffusion coefficient, D, in x, y, z direction is given as

D

=

Dx[1 +h/I(Dy/Dx} 1/2+h/n(Dz/Dx)1/2] ( 1.27) I

=

film length, n

=

film width, h

=

thickness Dx, Dy, Dz are diffusion coefficient in x,y,z directions.

For homogeneous materials, Dx

=

Dy = Dz so that

D = Dx [1 +h/l+h/np/2 (1.28)

Since Mt is a linear function of tl/2 the slope 8 of the plot allows the calculation of D and Dx, from Eqnl.28 and Eqn.l.29

D = [82h2n/16Ms)1/2 (1.29)

and

Dx = [8²h²n/16Msp/2[1 +h/l+h/n]2 (1.30) Diffusion coefficient can be estimated from permeability [31-34] Q, using the equation

D=Q/S (1.31)

Alternatively D can be calculated from the absorption vs time curve using the Eqn.1.29. Some general observations can be made from the above discussions. The amount of water taken up by a given sheet is proportional to the square root of time. The time required to reach a given stage of decomposition is proportional to the square of thickness of the sample. The percent increase'in weight after exposure to water for a definite period is inversely related to sheet thickness of sample. This method of determining diffusion coefficient at vapour pressures above O.7SRH is not reliable.

(27)

Introduction bsorption rates are observed to be higher than what the water Water a

ur Pressure increase suggests. Also at higher vapour pressures vapo

attaining equilibrium absorption, takes a long time.

The permeability or permeation coefficient (P) of a material depends on solubility (5) and diffusivity (D) of penetrant liquid through elastomer, and is a product of D and 5 [35]

P = D*S (1.32)

Many of the additives when used in the compounded rubber increase the solubility of water in the rubber. Also an increase in temperature usually increases both D and 5 in most polymers. An increase in cure usually decreases P.

The amount (Q) of the permeant (in mg) transported through the polymer membrane is given by relation [35]

Q = P(Pl.p2)At/L (1.33)

Here pland P2 are the vapour pressures (in cm Hg) of water on the wet side and dry side of the barrier respectively: A is the area (in cm2) of the membrane exposed to the permeant. L the thickness of the membrane, t is the time duration of exposure in sec. and P the permeability constant.

Thus it can be seen that the rate at which water is transported through the membrane depends on the water vapour pressure differential across the membrane, permeability constant and the membrane thickness.

1.1.6 Sealing properties

Elastomeric seals used in the deep sea applications demand special design considerations to meet challenges of operating environment, dynamic respox;se. Major issues include sealing efficiency, compression set, stiffness, and long term behavior. Studies on compression stress relaxation behavior under different environmental conditions enable estimation of

(28)

Chapterl

1.1.7 Acoustic properties

For a plane wave propagating in x direction through a homogenous elastic medium the wave equation is [7]

P

=

Po cos(kx-rot+~o) (1.34) P where po is the amplitude and is the initial phase. The total phase cl> at position x and time t is ~

=

kx - rot + <Po

The wave number, k, and angular frequency, 6), are related respectively to the wave length, A, and frequency f, of the wave are as follows:

k = 27t

A' ^Cl)

=

2nl

The complex number representation of the wave equation is given by ( 1.35)

(1.36) In a loss less medium sound waves propagate with constant sound speed. For a sound wave, which has the single wavelength, the sound speed and the frequency are related as follows

c = fA = 6)/k (1.37)

The increase in the wave length of sound with decreasing frequency has important implications for attenuation of sound.

An elastic wave can be launched in a material like rubber by applying a sinusoidally varying force into the material surface. A longitudinal wave is launched if the force applied is perpendicular to the surface. As this longitudinal wave propagates into it, the rubber molecules are forced back and forth by the oscillation of the wave. This gives rise to local pressure and density fluctuations. Wave properties such as, speed and attenuation are characterised in terms of corresponding modulus. Thus longitudinal wave is defined by complex elastic modulus, shear wa\'e by complex shear

(29)

Introduction modulus etc. Jarzynski [7] discusses the basic nature of sound propagation, the equations governing sound interaction in materials properties and gives an insight into the nature of sound absorption in the material. Under water acoustic transducers use rubbers in three major roles: Acoustic absorbers, Reflectors, and acoustic window materials. Acoustics absorbers are important for sonar transducers, acoustic baffles, acoustic calibration facilities and also for reduction of sound radiation and echoes from ships and submarines. Acoustic reflectors are used as decouplers as well as in sonar dome wedges. Acoustic window materials act as the coupling medium between the transduction material and water. Rho-c rubber [36] is a widely used material because its acoustic properties are close to that of water. When pc, the product of density of a material and the velocity of sound in the material is equal ~o that of the medium, the material is loosely called as Rho-c material. The product of density (p) and sound velocity(c) is the characteristic acoustic impedance. This parameter is the ratio of sound pressure to particle velocity. Sound energy transmission between adjacent media takes place without reflection losses if the acoustic impedance matches. But in a viscoelastic medium like rubber there is a certain measure of sound energy dissipation as the wave propagates through the solid.

Since acoustic waves are pressure waves supported by the particles of the medium, particles in the medium participate in the oscillatory motion;

sound wave propagation is directly linked to the density as well as elastic properties of the medium. If the acoustic impedance of the propagating medium is Zl

Zl = PICI

and that of the receiving medium

Z2 = P2C2

The intenSity of reflection

R

=

PI C1 - P2 C2 _ ZI - Z2 PICI + ^P1C2 ZI + ^Z2

(1.38)

(1.39)

(1.40)

(30)

Chapterl

Where acoustic wave is incident on viscoelastic material like rubber due to the dissipative mechanism within the material acoustic impedance gets modified as

where ac₂

r = - -

ill

(1.41)

Since acoustic impedance of rubber is a complex parameter, while it is possible to provide a perfect match for the real part of the impedance, there will always be a residual mismatch due to imaginary component.

Reflection free transmission of acoustic energy from rubber to water is not possible. However, by careful material selection and optimization, pressure reflection coefficient close to zero and pressure transmission coefficient close to unity could be achieved. The propagation of an acoustic wave through a solid polymer is determined by a number of parameters, which define the reflection, transmission and energy absorbing properties. Sound wave is incident at a boundary interface. Reflection and transmission occur as shown in Fig 1.3 The pressure reflection coefficient, Rp describes the reflective properties, correspondingly the pressure transmiSSion coefficient, Tp and acoustic power dissipation (PD). These relationships can be expressed as

PO

(1.42) (1.43) (1.44) where Pi. pr, Pt are incident, reflected and transmitted pressure respectively.

(31)

Introduction

Water

po Co Pr

Acoustic lining

Fig.l.3 Reflection and transmission of sound at a water polymer interface.

For efficient an echoic function bqth Rand T must be close to zero.

Using the terminology of echo reduction (ER) and transmission loss (TL) ER = -20 log Rp

TL = -20l0gTp

(1.45) (1.46) Transmission loss quantitatively describes the weakening of sound between a point 1 metre from the source and a distant point. Similarly echo reduction is the weakening of the intensity of reflected signal. In a lossless medium sound waves propagate with a constant speed. Speed of sound is related to the wavelength and frequency as per Eqn(1.37.)

It follows from Eqn.1.37 that the sound wave length is inversely proportional to the frequency. Typically an acoustic window material with sound speed around 1500 m/sec will have the wave length }.=15m at 1000Hz. Hence at audio and low ultrasonic frequency range the encapsulation thickness of 3mrn is small compared to wave length of sound and hence acoustic impedance mismatch is less severe. Sound absorption,a, in units of dB/ cm, is a measure of the loss in energy of the sound wave as it travels through the solid. The energy of the sound wave is converted into

(32)

Chapter 1

positions, expressed in decibels. In practice, because the polymer is

-

viscoelastic, sound speed ' c' is related to complex modulus. In an unbounded, isotropic solid there are two independent modes of acoustic propagation longitudinal and shear. In the longitudinal mode, the particle motion is'parallel to the direction of propagation, while in the shear mode. , the particle motion is perpendicular to the direction of propagation.

For characterizing a solid four parameters namely longitudinal sound speed CI, shear sound speed, Cs, longitudinal absorption, aI and shear absorption, as. One important experimental technique gives a modulus values rather than sound speed. The relationship is particularly simple [7]. Thus

c* = (G*

I

p)1/2 (1.47)

[37] The modulus and velocity may be expressed in complex form as in Eqn.l.14 and

c* = cl+iC2 (1.48)

Thus,

. . , (G'+iGtI)

~c )-

= -'----'--

(1.49)

P

=

^{C: -} c₂+ 2ic_lc2 (l.se)

Separating and identifying real and imaginary parts gives:

. 2 2

G =p(CI -C 2 ) (1.51)

and . [l+iac/w]

c - c ]

- [1+a²c²^/w2

(1.52)

Consideration of the normal complex exponential form for a progressive \va~e [38,39] allows one to relate Cl ami C2 to the observed phase velocity c and the amplitude of attenuation constant by the relations

(33)

Introduction

c (1.53)

and

(1.54)

• [l+iac/co]

C =C[1+a²c²/co²]

(1.55)

since G*= G+ i G" =[c*

/pF

^(1.56)

Equations can be used to solve for the magnitude of the phase velocity and attenuation.

Doing so yields

(1.57) and

(1.58)

Therefore by substituting the appropriate storage and loss modulus, C, K or E the acoustic parameters, sound velocity and attenuation can be estimated from dynamic mechanical properties

1.2 Underwater electro acoustic transducers

Electro

a~oustic

transducers are widely used in underwater surveillance and detection. In the civilian sector they include fish finders, sea boUo m pro f·l ¹ers, ocean depth sounders, and seismic exploration

(34)

devices while in the military sector applications ,they are found in mine 1 hunter~, torpedo nose cones, homing heads and anti submarine detection systems.

A transducer is a device that converts one form of energy into another. If a transducer converts mechanical energy into electrical energy and vice versa, it is called an electro mechanical transducer, a particular class of the electro mechanical transducer is the electro acoustic transducer.

Acoustic radiation in the form of an acoustic wave arises from the vibrations of the transducer, which is energized, by a voltage source. The transducer vibration of the transducer imparts a periodic motion to the particles of the medium in contact with it: the periodic motion of the particle of the medium about their equilibrium position along the line of energy propagation constitutes the acoustic wave. Such a transducer is said to function in transmission mode. When the sound wave is incident on a transducer the pressure variation in the medium in contact with the transducer surface imparts a vibratory motion into it. The mechanical energy is converted by the transducer into electrical energy, resulting in a voltage out put. Such a transducer is functioning in the reception are known as acoustic receivers, or hydro phones.

Transducers can be designed to operate at over wide range of frequencies starting from a few Hertz to several thousand Mega Hertz.

Frequencies between 20 Hz and 20 kHz is known as audio frequencies and those between 16 kHz and10⁹as ultra sonic frequencies.

Acoustic systems are operated with a wide range of power- from microwatts to kilov\'atts. The minimum acoustic power needed to be detected in a system is set by the thermal noise of the system or mediuIll·

The upper lim~t is set by practical design problem such as break down limit and mechanical strength in the transduction material. Maximum power that can be transmitted in water is only 0.33 watts per cm² without distortion.

24

(35)

Introduction

-

The geometry of a transducer varies depending upon the application.

be lanar spherical, or cylindrical. Its dimensions vary from a few It may p ,

t ^Sto a few meters. It is difficult to manufacture large transducers miIlime er

in single element. It is common practice to fabricate either mosaics of I paced elements or arrays with inner spaced elements of the close y s

required size and shape.

The majority of transducers fall into two categories-those which employ electric fields in their transduction process and those which employ magnetic fields. Some are inherently linear, while others have to be polarized to produce linear action. This arises from the fact that the force producing acceleration of the active mass of the transducer, which in turn causes acoustic rad~3tion, can be directly proportional to the square of the applied signal, depending upon the physical mechanism employed tor transduction.

1.2.1 Acoustic transduction

Under water electro acoustic transduction is accomplished by either of the two phenomena: Electrostriction and magnetostriction

Electrostriction refers to the conversion of energy between acoustical and electrical forms by means of a dependence between electrical fields and particle displacements in ferroelectric or piezo electric materials.

Magnetostriction denotes conversion of energy between acoustical and electrical forms by means of a dependence between magnetic fields and particle displacements in ferro magnetic materials. In the electrostriction phenomenon, there is a distinction between piezo electric effect and ferroelectric effect. Piezoelectric transducers use crystals in which the dimensions change according to the applied electric field. If the field is alternating, the crystals vibrate and give an acoustic radiation. Typical piezoelectr"" "

IC matenals used for this purpose are quartz, ammonium hydrogen phosphate, Tourmaline and Lithium sulphate.

(36)

Ferroelectricity is an electrical phenomenon analogous to

the'

ferromagnetic phenomenon. Ferro electricity can be defined as reversibility in a polar crystal, of the direction of the electric dipoles by means of art applied electric field. Most popular Ferro electrics or piezo electric ceralltic used in electro acoustic transducer fabrication are Barium Titanate and Lead Zirconate Titanate

1.2.2 Transducer Materials

Transducer materials can be broadly classified into two categOries namely active transduction materials and passive acoustic materials. Both are equally important in the efficiency and reliability of the transducer system performance. A brief discussion of these materials will be relevant in the context

Piezo- electric effect was discovered in 1880 by Jacques and Pierre Curie in quartz. It is based on the observation that a mechanical strain results from a voltage applied to a piezoelectrical material and voltage produced is proportional to the strain. The inverse effect was discovered in the following year by Lippman in quartz. It relates to the appearance 01 electric charges on the opposite surfaces of the cryst.'1I, proportional to the stress. This unique property of certain crystalline material has been exploited in the design of piezo electric transducers.

1.2.2.1 Active Materials

Piezoelectricity is the property possessed by some materials 0:

becoming electrically charged when subjected to a mechanical stress. Suer materials also exhibit the converse effect. i.e. the occurrence of mechanica deformation on application of an electric field. The piezoelectric effect wa~

first observed ,in naturally occurring single crystal compounds e.g. quart;

and Rochelle salt. The occurrence of piezoelectricity in such compounds

i~

due to the lack of a centre of symmetry in the unit cell and consequent!1 distortion of the unit cells produces electric dipoles

26

(37)

Introduction

-

Certain compounds can be made piezo electric by the application of a . h electric field (polarization), these are termed Ferro electric materials hig I of such materials are Barium titanate and Lead Zirconate Titanate

exaIIlP es .

which can be produced as single crystals or as poly crystalline aggregates by the ceramic process The polarization process involves the application of an electric field across the ceramic, usually at an elevated temperature, causing switching or realignment of the dipoles in the direction of the field.

After removal of the electric field there is a remnant polarization in the ceramic, which is responsible for its piezo electric properties. The resulting ceramic is now anifotropic and can be returned to its unpolarised iso tropic condition by raising its temperature above the Curie point or by mechanically over stressing.

These piezo electric materials are the actual energy converters used in the construction of underwater acoustic transducers. Poly crystalline Ferroelectric has been extensively used for fabricating transducer elements.

Most favored transduction material for underwater electro acoustic transducers are polarised ferroelectric ceramics like Barium Titanate and Lead Zirconate Titanate. The applications of ceramic transducers fall basically in two broad categories-High power sources and high sensitivity receivers; specific ceramic compositions are recommended for the two applications.

1.2.2.2 Passive Materials

There are a number of materials, other than active transduction materials, used in the construction of underwater electro acoustic transducer system. They are, in general, referred to as passive acoustic materials. Passive materials include a range of metallic and non-metallic materials with well defined and carefully controlled acoustic properties.

Th' elr role in the construction of the transducer are diverse, like sounder absorbers .

, acoustIc baffles, window materials, acoustic reflectors, de- COuplers ' . .

, acouStic fill flUids, seals, 0' rings etc.

(38)

~_, ~

Acoustic reflector materials are used in transducer arrays fOr reflecting and isolating the noise generated by the propellers. Acoustic reflectors are fabricated of rubber sheets into which holes have

been

moulded and open end covered. The air trapped in the holes provide the necessary compliance for effective low frequency isolation of underwater sound, reflector materials are also designed with syntactic foams and various micro balloons. The visco elastic polymer air micro bubbles composites are particularly useful in the design of anechoic coatings, Various inclusion-viscoelastic polymer matrixes have been investigated for use as sound absorbing material. Among the inclusions are sawdust [40], metal oxides, metal oxide micro particles, phenolic micro particles, metal powders [41] etc.

Acoustic baffles are used in large high performance underwater transducer arrays for the purposes of isolating ship's noise as well as for improving directivity and sensitivity of the transducer elements. The baffle forms a major element contributing to better performance of the system The materials used for construction of baffles are therefore required te possess specific acoustic and dynamic mechanical properties. As the application of baffles is in deep marine environment bdfle rubber muS!

also be compatible with marine water, temperature pressure and dynami(

loading condition. Rubbers show unique combination of stiffness an' damping capability. A major consideration is the constant modulus over wide range of the operating frequency. Frequency - modulus relationshif of SBR rubber has been studied to achieve an optimum combination (}

properties. Results of the study are reported in the present work.

1.3 Rubbers as passive acoustic materials

Rubber forms a major class of passive acoustic materials. Rubbe' components in underwater electro acoustic transducers rank as one of

t~

most sophisticated applications due to the additional functiO[l~

requirement namel\' propagation of acoustic waves. Because of

t~

(39)

Introduction . imposed by ocean environment coupled with application constralnts

. . formance requirements, conventional engineering rubbers can specific per

t the performance standards envisaged. There exists a need to hardly mee

d d develop application specific passive acoustic rubbers with stu y an

balance of performance and long service life. Good amount of information . 'lable in literature on the rubbers used in general underwater

IS aval

engineering applications. But in functionally specific cases like passive acoustic components used in underwater transducer technology there exists large gaps in information. The present study pays attention to such critical technologies. In these function-specific applications the rubbers are required to possess critical combinations of commonly referred engineering properties as well as desired acoustic properties. In several cases the intended underwater service life is of the order of a decade and more.

Rubber is extensively used in the construction of underwater electro acoustic transducers. The major applications include, transducer encapsulation, acoustic baffle materials, under water seals, junction box.

Primary function of an encapsulation material is the protection of e1ectro acoustic transduction devices from water. In this positive role encapsulation material should not adversely affect the acoustic transmission efficiency of the transducer. Because of the close acoustic impedance properties of rubber with water, ease of fabrication, sealing efficiency, and versatility to tailor m"ke properties; rubbers are the most favored material chosen for this application. However, the performance requirements reliability and service life demanded of the underwater transducers call for rubbers with more stringent performance specifications.

The important properties considered in the design of encapsulation materials for underwater electro- acoustic transducers are (1) Water absorption and 'permeation (2) Electrical resistivity (3) Dynamic mechanical properties (4) Acoustic impedance (5) Ease of processing and (6) Permanenc f

e 0 properties. Of the above parameters water absorption and

(40)

Chapter 1

permeation and consequent changes in other properties determine the service life of underwater devices

1.4 Scope of the study

~ Foregoing review brings out the need for function specific rubbers for optimum performance in underwater electro - acoustic transducers. There exist large gaps of information in the functional properties for specific application areas such as encapsulation rubbers, baffle rubbers, seal materials. The above rubbers must function in an operating environment characterized by dynamic stresses from cyclic mechanical forces, temperature variation, electrical field, and corrosion etc. Water ingress causes unacceptable changes in properties. Present investigations aim at filling gaps in the system and evolving compound design approaches for performance improvement and added service life. A major part of the work is devoted to studying the long term properties of the selected vulcanizates based on CR, BIIR and NBR with the aim of generating viable life estimation model.

1.5 Structure of the thesis

The thesis comprise of six chapters CHAPTER 1

This chapter introduces the topic of the work starting with a general introduction to the engineering applications of rubbers, discusses role of rubbers in underwater engineering. Chapter gives a description of underwater electro acoustic transducers, materials used in their construction and goes on to describe the role of rubbers as the major passive component. Chapter discusses in some detail the important

propertie~,

pertaining to the application of rubber as encapsulants, namely water ingress through diffusion and permeatl. n ^O•

viscoelasticity, dynamic mechanical testing, acoustic wave

Studies on Rubber Composition as Passive Acoustic Materials in Underwater Electro Acoustic Tranducer Technology and Their aging Characteristic

Thesis submitted to the

by

DOCTOR OF PHILOSOPHY

CERTIFICATE

"Studies

Preface

CONTENTS

n

Chapter'1

INTRODUCTION

=

=

"{ =

=

=

=

~

!

=

=

=

=

=

o

.

I

L::=o

Ihp

=

=

=

=

=

=

=

=

=

=

=

-

I

= -'----'--

=

/pF

a~oustic

-

the'

i~

-

~_, ~

been

t~

t~

propertie~,

^!